When mixing concrete, three principal ingredients shall be mixed: Cement, aggregate and water. In order to carry out this mixing operation, extensive use has been made of free-fall mixing machinery. A mixer of the type considered here has a horizontal mixing drum with two series of wings, viz. mixing wings and discharging wings. The drum is charged at one end, and the material then meets the mixing wings, which move the material in upwards direction and again lets it fall down, whereby the ingredients of the mass are mixed together. During this operation the wings are adjusted in such a way that the material is kept back at the input end of the drum. When the drum shall be emptied, its direction of rotation is reversed. The mixing wings then feed the material in a direction away from the input end towards the other end of the drum, when the material is discharged by means of the helicoidal setting of the discharge wings. Thus the horizontal position of the drum is maintained during the charging, mixing and emptying operations.